A Composite Variable Impedance Control Architecture with Adaptive Feedforward Assistance for Rehabilitation Exoskeletons

Abstract

Home-based rehabilitation exoskeletons often suffer from control instability due to low-cost force sensors. This paper presents a robust, sensorless Composite Variable Impedance Control architecture that separates trajectory tracking (virtual stiffness K) from active assistance (adaptive feedforward torque τassist). By eliminating high-frequency force feedback, the system ensures intrinsic stability. Experiments on the CURE platform demonstrate independent modulation of compliance (RMSE 2.64° to 15.90°) and effective assistance during simulated weakness, reducing tracking RMSE from 13.77° to 3.12°. Results show τassist contributes 59.6% of total torque, enabling "High-Assistance, High-Compliance" interaction without reactive stiffening. This provides a stable execution layer for advanced, bio-signal-driven "Assist-as-Needed" (AAN) therapies.